Silver halide-based photography is superior in photographic characteristics, e.g., photographic speed, facility of gradient control, etc., to other photographic systems such as electrophotography, diazo photography and so on, and has consequently been most generally used. In recent years, techniques have been developed which enable simple and rapid formation of images by changing the image-forming processing used in the silver halide-based photography from the conventional wet process using a developing solution or the like to a dry process using heat.
Heat developable photosensitive materials are well-known in the photographic art, and such materials and processes for their use are described in, e.g., Shashin Kogaku no Kiso, pp. 553-555 (Corona Co., 1979); Eizo Joho, p. 40 (April 1978); Nebletts Handbook of Photography and Reprography, pp. 32-33, (Van Nostrand Reinhold Company, 7th ed.; U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075; British Pat. Nos. 1,131,108 and 1,167,777; and Research Disclosure, pp. 9-15 (RD-17029) (June 1978).
Many methods for obtaining color images have been proposed. For instance, in the method of forming color images by binding couplers to oxidation products of developing agents, the combination of p-phenylenediamine type reducing agent and phenolic or active methylene-containing couplers is disclosed in U.S. Pat. No. 3,531,286, reducing agents of the p-aminophenol type are disclosed in U.S. Pat. No. 3,761,270, reducing agents of the sulfonamidophenol type are disclosed in Belgian Pat. No. 802,519 and Research Disclosure, pp. 31-32 (September 1975), and the combination of sulfonamidophenol type reducing agents with 4-equivalent couplers is disclosed in U.S. Pat. No. 4,021,240.
In addition, the method of forming positive color images using the light-sensitive silver dye bleach process, and useful dyes and bleaching methods are described in, e.g., Research Disclosure, pp. 30-32 (RD-14433) (April 1976), Research Disclosure, pp. 14-15 (RD-15227)(December 1976), and U.S. Pat. No. 4,235,957.
Moreover, the method of forming images by heat development using compounds containing a dye moiety and which are capable of releasing a mobile dye under high temperature in correspondence or counter-correspondence to the reduction reaction of silver halide to silver, is disclosed in European Published Patent Application Nos. 76,492 and 799,056, and Japanese Patent Application (OPI) Nos. 28928/83 and 26008/83 (the term "OPI", as used herein, refers to a "published unexamined Japanese Patent Application").
In these heat developable photosensitive materials, organic silver salts are usually incorporated as a source of silver ion supply. This is generally required because the materials typically cannot produce images of perceptible density, much less of photographically sufficient density, by heating unless organic silver salts are contained therein.
However, incorporating organic silver salts into heat developable photosensitive materials generally has the following disadvantages:
(1) Raising the heating temperature and prolonging the heating step with the intention of achieving sufficient density causes an increase in minimum density (fog density).
(2) Photosensitive materials in which both spectrally sensitized silver halides and organic silver salt oxidizers are present cause a sharp decrease in sensitivity during storage.
(3) It is difficult to flocculate organic silver salts in the washing step included in an emulsion-making process, because they have low specific gravities.